In-ground swimming pools add a wonderful touch of luxury, and more important, they are a cool place to hide from Australia's truly merciless summer heat. However, there are a myriad of structural, legal and safety concerns that come with excavating and installing an in-ground pool — so many that it's easy to overlook some aspects that may be considered less important by most. The type of soil you build your pool on might seem to be a lesser concern, but without proper research and planning, you can rapidly end up with a cracking, leaking disaster on your hands.
Expansive soils are one of the most common causes of damage to the foundations of any building, and the foundations of your shiny new pool are no less vulnerable. They are so dangerous because of their absorbency — depending on inclement weather and the level of the local water table, expansive soils can shrink or expand by as much as 30 percent, depending on how much moisture they absorb. As you can imagine, this can direct tremendous amounts of pressure on your foundations, causing them to distort and crack, taking the pool with it.
In almost all cases, the amount of clay content in your soil will dictate how expansive it is — clay is a silicate, with tremendously absorbent properties. If you think your property might be build on clay-rich soil, have a thorough and professional soil inspection conducted before building work commences. If it turns out that your soil is expansive, it's not the end of your swimming pool dreams, but extra precautions will have to be taken (the soil inspector, or your pool building contractors, will generally advise you on this themselves). These precautions may include:
- Extra underpinning
- Foundation piers
- Reinforced concrete foundations
- Pre-wetting your soil (this helps to trap moisture permanently within the clay and reduce its expansive properties)
Obviously soils high in salt can severely damage and corrode the metal underpinnings of a foundation, but they can also do significant long term damage to the concrete foundation itself. Because salt is a mild acid, it decreases the pH of the concrete it comes into contact with, causing the binding concrete paste to corrode and become porous. Groundwater, which contains small amounts of sulphuric acid, enters these pores, further damaging the structural integrity of the concrete and causing it to decay.
Whether or not high soil salinity will affect you depends largely on your area. Obviously, coastal areas have naturally higher salt levels in the soil, but parts of Western Australia and Victoria also have extremely high levels of salt salinity, largely due to land clearing efforts and poorly-implemented farm irrigation. Having your soil tested is the best way to test its salt levels — you can have a professional soil tester come over to inspect your ground, or you can send samples in the post to various remote testing companies.
In terms of what you can do to stop soil salinity damaging your pool foundations — not a great deal. The waterproof liner between your foundations and the soil will provide excellent protection, but it will eventually perish. Using extra-thick liners, or even doubling up on them, may prolong their life. Alternatively, you can add salt-resistant silicate solutions to your foundations to increase their resistance to salinity, but this can be difficult to achieve successfully with poured concrete.